1. Field of the Invention
The present invention relates to a variable capacitance element that includes a dielectric layer whose relative dielectric constant varies in accordance with an applied voltage, and in particular, relates to a variable capacitance element that includes such a dielectric film and that is provided on a piezoelectric substrate, and to a tunable filter that includes the variable capacitance element.
2. Description of the Related Art
To date, a variable capacitance element whose capacitance can be varied has been widely used in an electronic component. As such a variable capacitance element, Japanese Unexamined Patent Application Publication No. 11-260667 discloses, for example, a variable capacitance element in which a barium strontium titanate film is formed as a dielectric layer on a ceramic substrate.
Meanwhile, for mobile communication devices such as a cellular phone, there is a strong demand for reducing the size and weight of electronic components. In order to meet such a demand, piezoelectric devices such as a piezoelectric resonator and a piezoelectric filter are widely used.
In Japanese Unexamined Patent Application Publication No. 2008-54046 listed below, a piezoelectric filter illustrated in FIG. 19 is disclosed as an example of this type of piezoelectric device. A piezoelectric filter 1001 includes a series piezoelectric resonator 1002 and parallel piezoelectric resonators 1003 and 1004. An inductor 1005 is connected between the parallel piezoelectric resonator 1003 and the ground potential. Further, an inductor 1006 is connected between the parallel piezoelectric resonator 1004 and the ground potential. Furthermore, a bypass piezoelectric resonator 1007 is connected between a node between the parallel piezoelectric resonator 1003 and the inductor 1005 and a node between the parallel piezoelectric resonator 1004 and the inductor 1006. A variable capacitor 1008 is connected in parallel with the bypass piezoelectric resonator 1007.
FIG. 20 is a schematic plan view of the piezoelectric filter 1001, and FIG. 21 is a schematic cross-sectional view of a portion along the A-A line in FIG. 20. The piezoelectric filter 1001 includes a substrate 1009 that is formed of a silicon or glass substrate or the like. The series piezoelectric resonator 1002 and the parallel piezoelectric resonators 1003 and 1004 are formed of piezoelectric thin film resonators that are provided on the substrate 1009. For example, as illustrated in FIG. 21, the series piezoelectric resonator 1002 and the bypass piezoelectric resonator 1007 are configured as piezoelectric thin film resonators over cavities 1009a and 1009b that are provided in the substrate 1009.
Meanwhile, the variable capacitor 1008 for adjusting filter characteristics is formed of a lower electrode 1011, a dielectric layer 1012, and an upper electrode 1013 that are stacked in this order on an insulator layer 1010 that is provided on the substrate 1009. Japanese Unexamined Patent Application Publication No. 11-260667 indicates that the dielectric layer 1012 is formed of barium strontium titanate (BaxSr1-xTiO3) or the like.
In addition, Japanese Unexamined Patent Application Publication No. 2008-54046 discloses that the insulator layer 1010 is formed of silicon dioxide, silicon nitride, or the like, and the lower electrode 1011 is formed of the same material as the lower electrode of the piezoelectric thin film resonator, such as Mo, Al, Ag, W, Pt, or the like.
According to Japanese Unexamined Patent Application Publication No. 11-260667, since a variable capacitance element having a dielectric layer of barium strontium titanate formed on a ceramic substrate can be provided, a small-sized element with a large capacitance variation can be obtained.
However, a dielectric layer of barium strontium titanate could be deposited well only on specific substrates such as a ceramic substrate of alumina or the like or a semiconductor substrate. Accordingly, in a case where a piezoelectric substrate for, for example, a surface acoustic wave device, a boundary acoustic wave device, or a Lamb wave device is used, a variable capacitance element that includes a dielectric layer having good characteristics and formed of barium strontium titanate could not be formed on the piezoelectric substrate.
Further, the deposition temperature of BST is as high as 800° C. to 900° C. Therefore, if a BST film is to be formed on a piezoelectric substrate that is formed of a piezoelectric material having a low Curie temperature, there is a risk of the piezoelectric characteristics thereof being degraded. Further, there are also such problems as the piezoelectric substrate warping or breaking and the BST film peeling off.
According to Japanese Unexamined Patent Application Publication No. 2008-54046, the variable capacitance is formed of a layered body of the lower electrode 1011, the ferroelectric layer 1012, and the upper electrode 1013, and thus a variable capacitance structure portion can be reduced in size and in thickness.
Here, if the variable capacitance element disclosed in Japanese Unexamined Patent Application Publication No. 11-260667 is to be used in a piezoelectric device, an excellent BST film cannot be formed on the piezoelectric substrate, as described above. Therefore, as indicated in Japanese Unexamined Patent Application Publication No. 2008-54046 as well, a piezoelectric thin film resonator needed to be constructed by forming a piezoelectric layer on the substrate 1009 that is formed of a glass or silicon substrate, and then, the variable capacitance element needed to be constructed separately.
Thus, the piezoelectric device could not be reduced in size. Further, interconnect resistance occurred between the piezoelectric resonator and the variable capacitance element, degradation of the characteristics occurred due to a parasitic component, and in turn the characteristics of the obtained piezoelectric device were not sufficient.
It should be noted that, in a modification of a fourth preferred embodiment described in Japanese Unexamined Patent Application Publication No. 2008-54046, a tunable filter is disclosed in which a variable capacitance element formed of a surface acoustic wave resonator and a varactor diode is formed on a piezoelectric substrate. In this modification, the variable capacitance element is formed of the varactor diode and is different from a variable capacitor that includes the ferroelectric layer.